1. Field of Invention
The invention is directed to a thin-film battery and a method for making same. More particularly, the invention is directed to a new thin-film lithium battery having a novel electrolyte permitting a battery to be fabricated having greatly enhanced energy density and specific energy over conventionally available batteries. The invention is also directed to a novel cathode permitting a battery to be fabricated having significantly enhanced energy densities over conventionally available batteries.
2. Description of Prior Art
A battery is one of two kinds of electrochemical devices that convert the energy released in a chemical reaction directly into electrical energy. In a battery, the reactants are stored close together within the battery itself, whereas in a fuel cell the reactants are stored externally. The attractiveness of batteries as an efficient source of power is that the conversion of chemical energy to electrical energy is potentially 100% efficient although the loss due to internal resistance is a major limiting factor. This potential efficiency is considerably greater than the conversion of thermal energy to mechanical energy as used in internal combustion engines, which always results in heat transfer losses. Moreover, the additional disadvantages of contaminants emitted into the atmosphere as byproducts of incomplete combustion and dwindling availability of fuel supplies have intensified research into batteries as an alternative source of energy.
One limitation of conventional batteries is that they use toxic materials such as lead, cadmium, mercury and various acid electrolytes that are facing strict regulation or outright banning as manufacturing materials. Another limitation is that the amount of energy stored and/or delivered by the battery is generally directly related to its size and weight. At one end of the development spectrum, automobile batteries produce large amounts of current but have such low energy densities and specific energies due to their size and weight and such relatively lengthy recharge times that their usage as a source of propulsion is impractical. At the other end of the development spectrum, small, light, lithium batteries used to power small electronic appliances and semiconductor devices have much higher energy densities and specific energies but have not had the capability to be scaled up to provide the high energy for high power applications such as use in automobiles. Further, these small, light, lithium batteries have low charge-discharge cycle capability, limited rechargeability and, even where scaled down for microelectronics applications, size that frequently is many times larger than the semiconductor chip on which they are used.
Thin-film battery technology is foreseen as having several advantages over conventional battery technology in that battery cell components can be prepared as thin, e.g. 1 micron, sheets built up in layers using techniques common to the electronics industry according to the desired application. The area of the sheets can be varied from sizes achievable with present lithographic techniques to a few square meters providing a wide range in battery capacity. Deposition of thin films places the anode close to the cathode resulting in high current density, high cell efficiency and a great reduction in the amount of reactants used. This is because the transport of ions is easier and faster in thin film layers since the distance the ions must move is lessened.
Most critical to battery performance is the choice of electrolyte. It is known that the principle limitation on rechargeability of prior batteries is failure of the electrolyte. Battery failure after a number of charge-discharge cycles and the loss of charge on standing is caused by reaction between the anode and the electrolyte, e.g. attack of the lithium anode on the lithium electrolyte in lithium batteries. An extra process step of coating the anode with a protective material adds to the complexity, size and cost of the battery.
The power and energy density of a battery is also dependent upon the nature of the cathode. To achieve optimum performance, the open circuit voltage and current density on discharge should be as high as possible, the recharge rate should be high and the battery should be able to withstand many charge-discharge cycles with no degradation of performance. The vanadium oxide cathode of the present invention has a much higher capacity per mole than the crystalline TiS.sub.2 of prior art cathodes.
The present invention avoids the limitations of present battery design and provides a novel battery having application as a battery used with manufacture of semiconductor components and as a high energy, high current macrobattery with appropriate scale-up of the described processes. The present invention includes a novel electrolyte having a good conductivity but more importantly it has electrochemical stability at high cell potentials and requires no protective layer between it and the anode during battery fabrication or use. The present invention also includes a novel cathode having a microstructure providing excellent charge/discharge properties.